Aortic dissection is an uncommon but often lethal condition where the inner layer of the aorta separates from the outer layer, creating a double channel. The moving septum thus created disrupts the flow of blood to the legs and viscera. Additionally, the thin outer wall resulting from the delamination of the aorta often develops into an aneurysm that may eventually rupture.
Generally, a dissection starts by a tear involving the inner layer of the aorta that causes the inner layer to separate from the outer layer over part of the circumference of the aorta. Upon dissection, a new channel is formed between the separated outer and inner layers of the blood vessel wall of the aorta. This results in the aorta having two channels instead of one. The inner layer of the blood vessel wall that has separated is called the septum (or the flap) and separates the two channels.
One of the two channels formed in a dissection continues to function as a blood vessel, allowing the blood to flow through it. This channel is called the “true lumen.” The newly formed second channel through which blood also flows is called the “false lumen.” The true and false lumens communicate proximally through a proximal tear and distally through one or possibly various existing distal tears. In some dissections, there is no distal tear.
A dissection which involves the thoracic aorta is called a “thoracic aortic dissection.” There are two types. Type A dissections involve a dissection in the ascending aorta, while type B dissections involve any segment of the descending aorta. Type A dissections require immediate surgery. Management of type B dissections is the subject of controversy. Some doctors advocate temporary medical management, while others advocate immediate stent-grafting. In cases where the septum is blocking an opening of a major artery supplying the viscera or the leg, urgent surgery may be required, and may be limited to a resection of part of the septum (fenestration) providing communication between the true and the false lumens. In a few cases, this needed communication can be achieved by perforating the septum and enlarging the resulting perforation with a balloon (balloon fenestration).
A variation of the balloon fenestration involves inserting and advancing two wires through the false and true lumen to a proximal point, and linking the two wires in some fashion to cut the septum with the resulting wire loop. This variation lacks control of the site and of the length of the tearing maneuver. These attempts have been complicated by anecdotal reports of high pressure of the false lumen channel with collapse of the true lumen and by detachment and folding of the septum, obstructing the outflow of the aorta.
One of the wires should be advanced into the true lumen, the other into the false lumen. Entering the false lumen would be easy if both lumens reach down to the femoral artery puncture site. If the false lumen ends proximal to the femoral artery, the false lumen could be engaged by penetrating the orifice of communication between the true and the false lumen, or by puncturing the septum as distally as possible, inserting a wire in the false lumen and dilating the orifice with a balloon to allow the septotomy cutter to engage the septum at this point. Both wires should come out of the body through the same orifice, either a cut-down in the common femoral artery or a sheath inserted in it.
Catheter based cutting methods and tools for aortic dissections are found in Bliss, US 2011/0118769. Bliss primarily teaches use of a movable cutting blade, where a U-shaped or hooked tool penetrates a septum, then hooks and cuts the septum in a proximal to distal direction by retracting a portion or entirety of the catheter, or by distally translating the cutting blade relative to the septum. The cutting of a septum of a aortic dissection, in a proximal to distal direction, is problematic. This type of cutting, by the pulling of the septum in a distal direction, using a fixed or movable blade, may result in sudden collapse of the true lumen with catastrophic consequences. Safe division of the septum requires distal to proximal cutting.
Bliss does suggest one embodiment of a catheter based cutting device intended to operate in a retrograde manner (i.e., cutting the septum from distal to proximal along the aortic dissection). The embodiment has fixed, passive, cutting blade fashioned from a small, standard surgical knife (razor blade) intended to be pushed proximally against the septum by an inner tubular component (actuator) of the catheter. This passive cutting blade does not permit a controlled septotomy. The actuator acts as a telescopic mechanism by sliding inside an outer tubular component of the catheter. The coaxial catheter system extends the length of the catheter and creates substantial friction during advancement of the catheter within the blood vessel. The friction naturally increases with any increase in diameter of the two tubular components. A catheter with inner and outer tubular components functioning telescopically does not have the flexibility to navigate the bends of the arterial system and, if some bending has been achieved, the friction between the two tubular components would increase dramatically. Further, the embodiment disclosed includes a distal end having a fixed “Y” assembly. A stationary cutting blade (razor blade) is adhered between diverging ends of two tubular components (or fixed arms) in a Y-shape. The resultantly immobile, Y-shaped distal end is a substantially large assembly for introduction into (and maneuverability within) a blood vessel. Further, maneuverability and control of the cutting assembly at the septum, during septum cutting, and the required pushing of a fixed, passive blade against the septum, makes this embodiment inoperable.
What is needed is a catheter based cutting device operable in a retrograde manner, having a cutting assembly and cutting component that controls the site and length of the septum cut in both acute and chronic aortic dissection. In acute dissections, the device would equalize pressure between the true and the false lumens, potentially avoiding development of an aneurysm, as well as correcting malperfusion of the viscera or legs. Acute aortic dissections often require only a few centimeters of septum division to equalize pressure between the lumens, and to provide low resistance outflow of both lumens to avoid development of aneurysmal dilatation of the false lumen (e.g., 5-15 cm in the instance of cutting from the level of the aortic bifurcation to below the renal arteries). In chronic dissections, where different viscera may be perfused by either lumen, a cutting device is needed to convert the double lumen into a single one, where cutting the fibrous septum permits insertion of a branched endograft in a manner similar to that used in thoracoabdominal aneurysms. Also needed is a catheter based cutting device having no mechanical actuator, providing small profile construction, where the cutting component is an electrode or a fiber delivering energy that cuts the septum upon placement, without need for a mechanical push mechanism. As a result, the catheter can have a small diameter and increased flexibility which facilitates advancement of the catheter through a tortuous aorta.